Commonly, optical devices such as a liquid crystal display device, an organic EL (electroluminescent) device, and a solid image sensing device are provided with a protective film in order to prevent a device surface from exposing to a solvent or heat during a production process. The protective film is required not only to have high adhesiveness to a substrate to be protected and high solvent resistance but also to have excellent properties such as heat resistance.
In addition, when such a protective film is used as a protective film for a color filter that is used in a color liquid crystal display apparatus or a solid image sensing device, the film is required to have high transparency in order to maintain the transmittance of light that transmits through the color filter.
Meanwhile, in recent years, the introduction of a retardation material into a cell in a liquid crystal display has been studied for cost reduction and weight reduction. For such a retardation material, a material is typically used that is obtained by applying, aligning, and photo-curing a polymerizable liquid crystal solution. In order to align the retardation material, an underlayer film is required to be a material having alignment properties after rubbing process or polarized UV exposure. To address this, on an overcoat for a color filter, a liquid crystal alignment layer is formed, and then the retardation material is formed (see FIG. 1A). The formation of a film serving as both the liquid crystal alignment layer and the overcoat for a color filter (see FIG. 1B) will achieve great advantages such as cost reduction and reduction in the number of processes. Hence, there is a strong demand for a material serving as both the liquid crystal alignment layer and the overcoat.
Typically, for the overcoat for a color filter, an acrylic resin having high transparency is used. For dissolving the acrylic resin, glycol solvents such as propylene glycol monomethyl ether and propylene glycol monomethyl ether acetate; ester solvents such as ethyl lactate and butyl lactate; and ketone solvents such as cyclohexanone and methyl amyl ketone are widely used from the viewpoints of handling properties and coating properties. Such an acrylic resin is subjected to thermal curing or photo curing to increase heat resistance and solvent resistance (Patent Documents 1 and 2). The related-art thermosetting or photocurable acrylic resin has appropriate transparency and solvent resistance, but an overcoat composed of this kind of acrylic resin cannot achieve sufficient alignment properties even by the rubbing process or polarized UV exposure.
Meanwhile, the liquid crystal alignment layer typically employs a material composed of a solvent soluble polyimide or polyamic acid. It has been described that such a material is completely imidized by post-bake to obtain the solvent resistance, and exhibits sufficient alignment properties by the rubbing process (Patent Document 3).
It has been also described that an acrylic resin including a side chain having a photodimerizing moiety such as a cinnamoyl group and a chalcone group is subjected to the polarized UV exposure to exhibit sufficient liquid crystal alignment properties (Patent Document 4).